The start-up of an integrated drive generator involves the establishment of hydraulic unit continuity between the fixed and variable displacement hydraulic units of the hydraulic pump and motor assemblies of the integrated drive generator. To comply with many specification requirements this continuity must be accomplished early in the start-up cycle. During start-up at normal temperatures, this routinely occurs as soon as oil is supplied to the hydraulic units of the IDG. However, at extremely low temperatures a problem can occur which results in the delay or prevention of the aforementioned continuity, causing the integrated drive generator to violate the specification limits. In particular, the problem occurs that an electrical load on an airplane cannot be carried by the electrical generator immediately after start-up even though the aircraft engine reaches idle. The failure to establish hydraulic unit continuity also results in inadequate hydraulic pressure for lubrication of journal bearings which can lead to bearing failure.
An example of a prior art hydraulic pump and motor assembly for an integrated drive generator of an aircraft is illustrated in FIG. 1 of the drawings. As indicated therein, the hydraulic pump and motor assembly 1 comprises a variable displacement hydraulic pump 2 and a fixed displacement hydraulic motor 3. The pump 2 and motor 3 have respective cylinder blocks 4 and 5 which are arranged for rotation about a common axis A--A within a housing 6 on opposite sides of a stationary port plate 7 of the hydraulic pump and motor assembly. The port plate 7, at its outer flange is secured between respective portions of the housing 6 by bolts 8. The port plate 7 is formed with apertures 9 through which hydraulic fluid communication between the pump 2 and motor 3 is established during normal operation of the hydraulic pump and motor assembly. Helical coil springs 15 resiliently bias the cylinder blocks in the direction of the port plate 7.
The operation of the hydraulic pump and motor assembly in an integrated drive generator of an airplane involves transmission of torque from an engine of the airplane to an input 10 which rotates the input about the axis A--A. The cylinder block 4 of the pump is connected to a shaft of input 10 for rotation therewith. Pistons 11 within the cylinder block 4 of the pump 2 are displaced during this rotation an amount which is a function of the setting of a variable swash plate 12 of the pump. Hydraulic fluid under pressure from the pump 2 is delivered to the hydraulic motor 3 through the port plate 7 for rotating the cylinder block 5 and an output shaft 13 to which it is fixedly connected. The swash plate 14 of the motor 3 is fixed so that the operating speed of the motor 3 is a function of the displacement of the pump 2. The rotary output from output shaft 13 is added to or subtracted from the rotary motion from the engine through the conventional differential gearing of an integrated drive generator for operating an electrical generator at a substantially constant rotational speed. That is, since the speed of the rotation from the airplane engine to the input 10 of the hydraulic pump and motor assembly will vary, the position of the variable swash plate 12 is adjusted in response to these detected speed variations for providing the necessary reduction or increase in this speed for obtaining the desired constant output speed to the generator. During normal operation there is a hydrostatic balance of the cylinder blocks and port plate.